Carbohydrate and Lipid Metabolism During Prolonged Aerobic Exercise in Athletes:university of W.a. Perth Wa Australia

My interest in comparative aspects of the extremes of primary metabolism dates back to 1975, and due to the excellent facilities for studying human exercise at the University of W.A., it seems expedient that in my present position I should turn my attention to the primary metabolism of humans.

A two hour marathon constitutes a period of extremse stress. Oxygen uptake rises by 24-fold, 2% of the total body energy reserves are consumed ,and the body is propelled over 48km . This prologed period of exercise is powered aerobically and blood lactate rises only minimally. The metabolic interactions during a successful marathon are by necessity complex and finely turned, and provide insights into the potential of the primary metabolic system when an organism is faced with an extreme situation. Also of interest however ,are the failings of the system in inexperienced runners who are unable to complete the run.

Metabolically the marathon runner faces a conundrum ; a carbohydrate-based metabolism can supply ATP at the highest rate, but carbohydrate stores are limited. Fat supplies are unlimited in terms of a 2-hour marathon, but in this case ,the rate of ATP supply suffers.

All runners, trained or untrained ,use a mixture of carbohydrate and fat as fuels during exercise. The trained athlete however ,uses a higher percentage of fat than does the untrained athlete. This strategy (a) ensures that the rate of ATP production never becomes limiting, and (b) spares liver glycogen for the blood and brain. These effects of training manifest themselves at different levels,e.g. Low RQ values and increases in muscle lipoprotein lipase but the precise metabolic perturbations,produced by training, which allow the fat enrichment of the fuel mixture, are unknown.

The untrained or unsuccessful runner uses a higher percentage of carbohydrate in the fuel mixture and therefore runs the risk of exhausting the body’s carbohydrate stores during the run. Muscle glycogen stores certainly can be exhausted within two hours during aerobic exercise and it has been suggested that the ‘wall’ encountered by inexperienced runners ,as well as the dizziness and disorientation experienced , are the outward manifestations of the exhaustion of body glycogen reserves. But again ,changes which either result from ,or compensate for , this glycogen depletion, have not been characterized.

I hope you all like this story and let me know what you think of it.

George T Saviel

16 August 2010.